1. Field of the Invention
The invention relates to a method of operating a heat exchanger apparatus comprising at least two heat exchanger units arranged in series, each unit having at least one vertical tubular duct for a first heat exchanging medium containing fluidisable granular material and a compartment for a second heat-exchanging medium through which said duct extends.
2. Description of the Prior Art
A heat exchanger apparatus of the type described above is disclosed in for example published Dutch patent application No. 77 03939 (GB No. 1,592,232). A problem arises in the use of such an apparatus when the first heat exchanging medium is a liquid which, as a result of the temperature change occurring in the heat-exchange process, creates heavy dirtying of the ducts through which it passes.
This is the case, for example, with liquids containing albumen, the albumen coagulating as the temperature rises. It is alternatively possible for a liquid to have a component which crystallizes out as the temperature is lowered.
A heat exchanger where a fluidised granular material is present in the vertical tubular ducts is in itself particularly suitable to withstand such dirtying. The intensive movement in the fluidised granules exercises a light scouring effect on the duct wall, so that substances sticking to it are removed. In practice, it has in fact been found that the fluidised material can certainly remove all kinds of deposits from the heat-exchanging surface and is therefore suitable for applications where heavy pollution may arise. This means that the heat exchanger does not have to be cleaned very frequently with chemicals or that the heat-exchanging elements do not have to be frequently dismantled so that they can then be cleaned mechanically and/or chemically.
Nonetheless, it has been found that applications exist for the exchange of heat between two media where the first heat-exchanging medium causes such intensive dirtying of the installation that even with the fluidised granular bed continuously in use excessive deposits occur in the vertical ducts.